are you sure your php can create /tmp/xcache/cache? try to disable xcache and make a php script <?php touch("/tmp/xcache/cache") ?> and see if it creates the file

The xcache directory and (xcache/)cache file both belong to the lighttpd user which is used by the lighttpd. I initially touched the "cache" file and chown'ed it. When I tail/less it now, it contains some "mambo jambo". So it looks like it's used indeed.

I created the following script (because I did not want to mess with the cache on runtime):